Ammonia adsorption has been studied on an Ir(100) surface in the temperature range 100-410 K. In contrast to previous studies on Ir(I 11), approximately 12% of the chemisorbed ammonia undergoes stepwise decomposition at 200 K. However, decomposition has been found to be an activated process wherein a difference in the activation energy of dissociation and desorption is estimated to be 21 kJ/mol. Recombinative nitrogen desorption, occurring at temperature as low as 500 K, has been found to be the crucial step for having continuous and efficient ammonia decomposition with an activation energy of 64 kJ/mol. Coadsorption of hydrogen and ammonia have been carried out to understand the partial pressure dependences for ammonia decomposition --- 0.9 +/- 0.1 with respect to ammonia and -0.7 +/- 0.1 with respect to hydrogen. Coadsorption data indicate that the negative order with respect to hydrogen is due to enhancement of the reverse reaction (NHx, + H --> NHx+1, x = 0-2) as well as reduction in the desorption temperature of ammonia in the presence of excess H-atoms on the surface. In contrast, coadsorbed oxygen acts as a promoter for the ammonia dissociation and leads to 100% ammonia conversion. The differences in the decomposition behavior with respect to the previous results for Ir(l 11) are indicative of the structure sensitivity of the reaction.